Long-term records of reservoir storage provide an integrated observational view of regional hydrological variability in managed systems and are directly relevant for water-resource management and drought risk assessment at regional scale. Here we analyze daily records of stored water volume percentages for major reservoirs in Catalonia over the period 1960–2026, using exclusively observational data provided by the Catalan Water Agency. After physically consistent preprocessing, we construct a set of annual metrics, including the annual mean stored volume percentage and the fraction of days below defined low-storage thresholds. Pairwise correlation analyses reveal a clear contrast between intra-basin and inter-basin behavior and show a pronounced increase in inter-basin synchronization after the late 1980s, particularly for low-storage and extreme drought metrics that are most relevant for water scarcity conditions. To assess whether this increased synchronization reflects emergent inter-basin coupling or the dominance of a common large-scale signal, we explicitly remove the leading regional mode of variability using a principal-component-based common-mode analysis applied to the annual reservoir metrics. After removal of this dominant mode, inter-basin correlations are strongly reduced in both early and late periods, and the apparent pre/post contrast is substantially weakened. The analysis provides robust observational evidence that inter-basin correlations strengthen after the late 1980s and that this change is largely captured by a single dominant regional common mode (PC1) in the annual reservoir metrics. Residual inter-basin structure beyond this leading mode is comparatively weaker and does not exhibit the same systematic pre/post amplification. These results indicate that reservoir systems that were historically only weakly coordinated now behave increasingly as a single regional system dominated by a common large-scale signal, particularly during drought-relevant conditions. This has direct implications for water management, as it limits the effectiveness of spatial diversification and inter-basin compensation strategies. No causal attribution is attempted; the results instead provide a quantitative observational baseline for future studies combining reservoir data with climatic and operational covariates.